In order to find the values of the labeled angles, you take into account that such angles are opposite angles by the vertex, thus, they both are equal and you have:
[tex]4x=75-x[/tex]you solve the previous equation for x, just as follow:
4x = 75 - x add x both sides
4x + x = 75
5x = 75 you divide both sides between 5
x = 75/5
x = 15
Next, you replace this value of x into the expressions for the angles:
4x = 4(15) = 60
75 - x = 75 - (15) = 60
Hence, the searched angle is 60°
